The use of a differential piston for elevators is known in the prior art. See, for example, U.S. Pat. No. 454,872 issued June 30, l891 to Otto Krell. A differential piston comprises a hollow piston at the upper portion of the assembly, the hollow piston being of lesser diameter than the cylinder which defines the lower portion of the assembly. The upper end of the hollow piston is attached to a superstructure above the elevator and the lower end of the hollow piston has an annular bore extending between the hollow interior of the piston and the interior of a cylinder which defines the lower portion of the differential piston. A sealing flange is fixed on the lower end of the piston beneath the annular port and frictionally engages the inner surface of the cylinder. Hydraulic fluid introduced into the top of the hollow piston flows through the annular port above the sealing flange and into the cylinder within which the sealing flange of the piston is received. The hollow piston remains stationary at all times and the cylinder reciprocates relative thereto. As hydraulic fluid is introduced into the hollow piston and enters the cylinder through the annular port, the pressure of the build-up of fluid between the inner surface of the top wall of the cylinder and the sealing flange on the piston causes the cylinder to move upwardly relative to the sealing flange thereby shortening the overall length of the assembly. The cab of an elevator is attached to the lower end of the cylinder so that the elevator rises upon the introduction of hydraulic fluid into the hollow piston to shorten the overall length of the differential piston.
The opening of a valve to remove hydraulic fluid from the cylinder causes the cylinder to move downwardly relative to the piston, thereby lengthening the differential piston and causing the elevator to descend.
One obvious advantage of such an arrangement is that it is not necessary to dig a hole in the ground to receive the cylinder of a hydraulic ram and cylinder.
U.S. Pat. No. 3,650,356 issued Mar. 21, 1972 to Richard L. Brown discloses the use of differential pistons to actuate elevators. Brown uses two laterally spaced differential pistons for raising and lowering the elevator cab, whereas Krell uses only a single differential piston for the elevator. The use of two differential pistons is preferable because it gives greater stability to the cab than does a single differential piston. However, according to Brown each of the differential pistons passes through the interior of the cab and through the same transverse plane of the cab, thereby taking away from the usable area of the cab and contributing to the instability of the cab.
Difficulty has also been experienced in the operation of the Brown elevator because of the inaccurate and unreliable registry of the cab with the floor levels.